The invention relates to a semiconductor device having an avalanche diode for detecting radiation, comprising a semiconductor body having a semiconductor layer structure of a first conductivity type, which layer structure comprises successively at least a first, low-doped semiconductor layer of substantially homogeneous doping, a second semiconductor layer having a doping concentration which is higher than the doping concentration of the first layer, and a third semiconductor layer having a lower doping concentration than the second layer, said layer structure comprising on one side a first contact layer which forms a nonrectifying junction with the first semiconductor layer and comprising on the other side a second contact layer which forms a rectifying junction with the last semiconductor layer of the layer structure.
The invention also relates to a method of manufacturing the device.
A semiconductor device of the kind described is known from Philips Technical Review, vol. 36 (1976) pp. 205-210.
Semiconductor devices of a variety of natures may be used for detecting radiation. The radiation may be both of a corpuscular nature and of an electromagnetic nature. Although in this application the detection of electromagnetic radiation will mainly be described, the device according to the invention is not restricted to the detection of electromagnetic radiation. In principle the device may be used for the detection of any type of radiation which can generate electron-hole pairs in a semiconductor crystal lattice by absorption of energy.
For the detection of electromagnetic radiation, radiation-sensitive resistors, diodes and transistors usually referred to respectively as photoresistors, photodiodes and phototransistors, are used. Which of these devices is to be preferred for a given application is determined by various factors, inter alia the response time, the quantum efficiency, the noise properties and the amplification desired. For many applications the so-called avalanche photodiode is preferred due to its internal amplification and its very short response time. This is the case in particular in optical communication systems.
The ordinary photoavalanche diode has a PN junction which is biased in the reverse direction to such a voltage that avalanche multiplication of charge carriers which are generated by incident radiation in the depletion zone occurs. The field strength distribution over the successive semiconductor layers, however, is comparatively unfavorable. With the minimum thickness of the depletion zone which is required to achieve a reasonable quantum efficiency (for example, for radiation having a wavelength of approximately 0.9 micron in a P.sup.+ PN.sup.+ silicon diode approximately 25 microns), the voltage across the diode is consequently comparatively high, which is undesirable in many applications.
It has been endeavored to improve this unfavorable condition by using a N.sup.+ P.tau.P.sup.+ (or P.sup.+ N.nu.N.sup.+) structure, wherein .pi. and .nu., respectively, denote very low doped P and N types. As a result of this the field distribution is changed so that with the same thickness of the depletion zone and with the same value of the maximum field strength the overall voltage across the diode is considerably lower than in a P.sup.+ PN.sup.+ diode. A disadvantage in an N.sup.+ P.pi.P.sup.+ diode, however, is that the avalanche multiplication which takes place in the P region, which for technological reasons usually is comparatively thin, must occur over a sufficient width so as to achieve the desired multiplication. Consequently the maximum field strength becomes high resulting in a higher noise factor.
In order to improve this, a N.sup.+ PP.sup.+ .pi.P.sup.+ structure has been suggested, see the already-mentioned publication in Philips Technical Review. Using this structure the rate of change of field strength across the PP.sup.+ region becomes less and the avalanche region becomes wider so as that a reasonable avalanche multiplication can be effected with lower values of the maximum field strength. With this structure a very low noise factor can be obtained.